1. Field of the Invention
The present invention relates to a semiconductor device.
2. Description of Related Art
In Japanese Patent Application Publication No. 2009-194065 (JP 2009-194065 A), a semiconductor device that is formed in a silicon carbide substrate that has a front surface on which a front surface electrode is provided and a back surface on which a back surface electrode is provided is disclosed. The semiconductor device includes a drain layer of a first conductivity-type that is electrically connected to the back surface electrode, a drift layer of a first conductivity-type that is placed on the side of the front surface of the silicon carbide substrate relative to the drain layer and has a lower impurity concentration than the drain layer, a base layer of a second conductivity-type that is placed on the side of the front surface of the silicon carbide substrate relative to the drift layer and is electrically connected to the front surface electrode, a gate electrode that is located in a trench that extends from the front surface of the silicon carbide substrate into the drift layer and is insulated from the silicon carbide substrate and the front surface electrode by an insulating film, a source layer of a first conductivity-type that is provided between the base layer and the front surface electrode and is in contact with the insulating film for the gate electrode and the front surface electrode, and a buried layer of a second conductivity-type that is provided between the drift layer and the base layer and is formed such that the depth of its drift layer side end from the front surface of the silicon carbide substrate is greater than the depth of a distal end of the trench from the front surface of the silicon carbide substrate.
In the above semiconductor device, when a voltage is applied between the gate electrode and the front surface electrode as a source electrode, an N-type channel is formed in an area of the base layer in contact with the gate insulating film. When a voltage is applied between the back surface electrode as a drain electrode and the front surface electrode as a source electrode in this state, a large current flows from the back surface electrode toward the front surface electrode. The magnitude of the current that flows at this time depends on the magnitude of the voltage between the gate electrode and the front surface electrode. In other words, the above semiconductor device functions as a trench-gate type N-channel vertical MOSFET.
In the above semiconductor device, a buried layer is formed between the drift layer and the base layer, and the depth from the front surface of the silicon carbide substrate to the drift layer side end of the buried layer is greater than the depth from the front surface of the silicon carbide substrate to a distal end of the trench. With this configuration, electric field strength in the vicinity of the distal end of the trench that may occur when a voltage is applied can be relaxed. This improves the voltage resistance of the semiconductor device.
The semiconductor device as described above is produced by the following procedure. First, a drain layer and a drift layer are formed in a silicon carbide substrate, and ion-implantation is performed into the front surface of the drift layer to form a buried layer. Then, a base layer is formed on the front surface of the buried layer by epitaxial growth, and ion-implantation is performed into the front surface of the base layer to form a source layer. In addition, a trench is formed from the front surface of the silicon carbide substrate, and a gate electrode is formed in the trench. Then, a front surface electrode and a back surface electrode are formed on the front surface and the back surface, respectively, of the silicon carbide substrate.
When a buried layer is formed by ion-implantation as described above, the buried layer suffers from minute defects, such as threading dislocation. When epitaxial growth is used to form a base layer on the front surface of the buried layer as described above, the defects of the buried layer may be transferred to the base layer and the base layer may also suffer from the same defects as those of the buried layer. When the base layer has defects, a leak current is generated when a voltage is applied between the drain and the source.